Humidity, Radiative and Surface-Flux Feedbacks on the Multiscale Organization of CRM-Simulated Tropical Convection

Abstract:

Positive feedbacks between column humidity, reduced radiative cooling and enhanced surface fluxes promote convective self-aggregation in limited area cloud-resolving model (CRM) simulations over uniform sea-surface temperature (SST). Near-global aquaplanet simulations with 4 km horizontal resolution and no cumulus or boundary-layer parameterization are used to test the importance of these feedbacks to realistically organized tropical convection. A 20480x10240 km equatorially centered channel with latitudinally varying SST is used. Realistic midlatitude and tropical cloud structures develop (see attached image). The natural zonal variability of humidity and convection are studied in a 30 day control simulation. A small white-noise humidity perturbation is then added to explore temporal perturbation growth. Atmospheric column budgets of moist static energy (MSE) quantify its covariability with precipitation, surface heat flux and radiative energy loss. Zonal Fourier analysis partitions these budgets by length scale.

Radiative feedbacks on MSE natural variability and perturbation growth are found to be positive, broadly similar across scales, and comparable to limited-area CRMs, capable of e-folding a column MSE perturbation in 10 days. In contrast, in the presence of horizontal SST gradients, synoptic-scale dry intrusions with enhanced surface latent heat fluxes damp tropical MSE perturbations and inhibit aggregation. Over sub-10-day timescales, dynamically-driven feedbacks dominate. The tropics and midlatitudes have similar timescales for loss of large-scale deterministic predictability.

This work is under review:

Bretherton, C. S., and M. Khairoutdinov, 2015: Convective self-aggregation feedbacks in near-global cloud-resolving simulations of an aquaplanet. J. Adv. Model. Earth Sys., submitted 6/2015.